1. Field of the Invention
The invention concerns a process for processing pulses delivered by a gamma camera and a gamma camera using this process. It concerns scintillation cameras or gamma cameras for instance of the ANGER type the American U.S. Pat. No. 3,011,057 of which describes the operating principles and its means of achievement. These gamma cameras are intended to detect and display photons emitted by radioactive bodies.
Gamma cameras are used in nuclear medicine to display in an organ the distribution of molecules marked by a radioactive isotope that has been injected into a patient. The use of gamma cameras can be extended to studying all radioactive bodies.
The operating principle of the gamma cameras is shown on FIG. 1. A radioactive body 1 emits gamma photons .gamma.. These photons are emitted from a point of the body 1, potentially in all directions. A scintillator 2 receives these gamma photons .gamma. and at time of impact converts each gamma photon .gamma. received into a light source s. A set of photomultiplier tubes 3 arranged opposite the scintillator 2 converts into electric signals the light radiation delivered by the source s in the scintillator. Each photomultiplier tube delivers an electrical signal dependent on the total quantity of light that it has received. Weighting arrays 4 (five in general), consisting for instance of impedances of same type convert the signals from the various photomultiplier tubes into five signals x.sup.+, x.sup.-, y.sup.+, y.sup.- and w. Signals x.sup.+, x.sup.-, y.sup.+ and y.sup.- contain information relevant to the X and Y positions of the light distribution barycentre on the detection plane consisting of the photomultiplier tube 3 input faces. The signal w represents the total energy recovered on the set of photomultiplier tubes 3.
A position calculation circuit 5 recovers the signals x.sup.+, x.sup.-, y.sup.+ and y.sup.- to integrate them over a duration equivalent to the duration of a scintillation and to determine the X and Y position of the impact. This position calculation circuit 5 delivers two signals x and y proportional to the X and Y position. An impact detection circuit 6 receives the signal w and determines as a function of this signal w representative of the energy received by the set of tubes 3 a validation signal v which indicates if the impact is to be taken into account or ignored. A display device 7 receives the signals x, y and v and displays or records, if applicable, the X and Y position impact point, an image consisting of the accumulation of impact points. Other measuring devices, without weighting arrays, can be considered. Their principles are to digitise the signals at the output of each photomultiplier tube and to use high-speed microprocessors to make the equivalent calculations. Likewise, the photomultiplier tubes can be replaced by semiconductor devices.
In reality, the radioactive body I emits a certain number of gamma-ray photons .gamma. in a given interval of time commonly known as a number of counts emitted per second. The number of counts emitted per second depends on the activity of the radioactive body 1. In addition, these counts are emitted in a random manner from any point of the radioactive body 1 and in any direction. In fact, to obtain an image, by projection in two dimensions, representative of the radioactive body 1, only the gamma-ray photons .gamma. coming from one direction, for instance normal to the scintillator 2, are of interest. For this, a collimator 8 can be used to collimate the gamma-ray photons .gamma. emitted by the radioactive body 1 on the scintillator 2. This collimator can consist, for instance, of a lead plate several centimeters thick drilled with a multitude of holes oriented for instance perpendicularly to its surface in order to let through only gamma radiations normal to this surface. The effect of adding a collimator is that a sort is made. Indeed, only the photons arriving perpendicularly to the collimator in the example pass through it. This represents one count from around ten thousand.
The counts last only for a very short time and the impact associated with the count is compared to a Dirac function over time. However, the impact on the scintillator 2 triggers a cascade of phenomena through the scintillator 2, the set of photomultiplier tubes 3, the weighting arrays 4 and the connections with the position calculation circuit 5 and the impact detection circuit 6 and finally gives birth to a relatively stretched pulse. To locate the impact, we integrate, into the calculation circuit 5, over a duration representative of the stretched pulse, the signals x.sup.+, x.sup.-, y.sup.+ and y.sup.- so that we can calculate that which corresponds to a barycentre of the light spot received by the set of photomultiplier tubes 3. But, if two counts are very close to each other, the stretched pulses may overlap thus falsifying the impact position calculation.
The aim of the impact detection circuit 6 is to determine whether an impact is to be taken into account. As detection is made at the same time as the impact position calculation, information is supplied to the display device which validates the position associating an energy level with it. Formerly, detection was processed thus:
detection of the exceeding of a noise threshold by signal w, PA1 integration of signal w from this detected threshold, PA1 detection of the maxima on this signal w, PA1 detection of the return of signal w to below the noise threshold thus putting an end to the integration of signal w, PA1 generation of a validation signal as a function of the signal w integration result if also signal w is between an authorised maximum and minimum, if only one maximum was detected when signal w was above the noise threshold. PA1 the gamma camera of FIG. 1 using a collimator does not allow the measurement of more than several tens of thousands of counts per second on a patient even though, technologically it is quite possible of accepting 200,000 counts per second; it also requires the injection of a highly active isotope into the patient, PA1 the gamma camera in FIG. 2 using a coincidence detection system allows a significant reduction in the doses of radioactive products to be absorbed by the patient by slightly reducing the number of counts taken into account (doses 100 times less for an equal number of counts), nevertheless, as we saw in the above mentioned article, with high activities of around 2 million counts, more than 30% of the usable counts are lost; such a system allows the recovery of around 14,000 counts out of 2 million, also the spatial resolution is in general better than with collimated systems. PA1 it is produced at least one compressed signal, each compressed signal being representative of one of the electric signals on which the duration of the pulses has been shortened; PA1 it is calculated the coordinates of the impact point from the compressed signals; PA1 it is detected the pulses on at least one analogue electric signal of each detector; PA1 it is produced a coincidence signal if a pulse is detected, in a time window associated with the simultaneity, on at least two of the analogue electric signals delivered respectively by the two detectors.
In fact, a detection such as this amounts to checking that the waveform of the energy received corresponds to a certain envelope. This envelope is used to check that there was only one pulse by the detection of a single maximum in a given energy range. Therefore, the low-energy pulses due to the Compton gamma-ray photon diffusion are also excluded.
2. Description of the Related Art
Good control over digital techniques has allowed the system to be improved. In patent request EP-A-0 470 909, a system is described which digitises the signals x.sup.+, x.sup.-, y.sup.+, y.sup.- and w. Then, using a digital filter, the transfer function of which is calculated to be the inverse of the transfer function of the items which convert a photon impact into an electric signal, these signals x.sup.+, x.sup.-, y.sup.+, y.sup.- and w are converted. The digital filtering greatly reduces the pulse duration of the signals. Before filtering, the duration of a pulse is slightly greater than one microsecond. After filtering, this duration may be lower than 250 ns (in fact, the duration depends, among other things, on the sampling frequency and the parameters of the digital filter). As the pulses are shorter, the number of pulse overlaps is lower than before. Under these conditions, more counts can be counted per unit time and, in the end, the images can be acquired more quickly. Detection is nevertheless ensured by an envelope system.
A system with collimator using digital filtering can be used to count up to 200,000 counts per second, which represent an emission of around 2 billion counts per second which corresponds to around 50 to 60 mCi. However, in nuclear medicine, making the patient as radiocative as this with the conventionally used isotopes incurs certain risks for the patient. In general, collimated systems are used with patients to process at most several tens of thousands of counts per second.
Also, we know that position (or positron) emitting isotopes exist where the gamma-ray photons are emitted in pairs in opposite directions, fluor 18 for instance. Another technique for collimating gamma-ray photons, in the case of positions, consists in using a gamma camera with a second detector and a coincidence detection circuit which checks the direction from which the photon emitted in the other direction comes. In general, the gamma camera uses two identical detectors. The coincidence detection circuit is connected to the two detectors of the gamma camera. FIG. 2 shows such a system.
On FIG. 2, a radioactive source 10 emits gamma-ray photons .gamma.1 and .gamma.2 in pairs in opposite directions. A scintillator, 11 and 12, is placed on each side of the radioactive source 10 to convert the energy of the gamma-ray photons .gamma.1 and .gamma.2 into light energy. We can see that such a system does not use collimators. The light energy emitted by each of the two scintillators 11 and 12 is received by two sets of photomultiplier tubes 13 and 14 respectively. The sets of photomultiplier tubes 13 and 14 are connected to weighting array assemblies 15 and 16 respectively. The assemblies 15 and 16 each transform the energy recovered from the sets of photomultiplier tubes 13 and 14 respectively into five signals. Four signals are used to locate the impact of the gamma-ray photon .gamma.1 or .gamma.2 on the scintillator 11 or 12, a fifth signal (W1 or W2) being representative of the total energy given off by the gamma-ray photon .gamma.1 or .gamma.2. The respective position calculation circuits 17 and 18 each recover the four signals, from the weighting arrays 15 and 16, which are representative of the position of the impact to calculate the coordinates of the impact point and to transmit them to a display and storage device 19. Another calculation circuit locates the origin of the two gamma radiations .gamma.1 and .gamma.2. Variants are possible at detector level; the signals can be digitised at the output of the photomultiplier tubes making the arrays unnecessary; the multiplier tubes can also be replaced by equivalent devices.
The impact detection circuits 20 and 21 recover the signal W1 and W2, respectively representative of the total energy from the array assemblies 15 and 16 respectively. These impact detection circuits 20 and 21 will, on the one hand, check that the count is valid, that is in compliance with the envelope and, on the other hand, calculate the power integral for each valid count. A validation signal, if the count is valid, will then be transmitted to the display device 19 and to a coincidence detector 22. If two valid events arrive at the same time, that is in a time window of for instance around 10 nanoseconds, the coincidence detector 22 will emit a binary signal to indicate to the display device 19 that there is indeed a coincidence. The display system 19 can reconstruct the paths of the photons and then the images.
With a patient, the gamma-ray photon emissions are made in all directions sometimes at the same time. Under clinical conditions, the detectable coincidences in general represent only 1% of the counts received by the scintillators 11 and 12. 99% of the counts received are single, that is, only one of the two photons emitted touches a gamma camera detector. In practice, gamma cameras using coincidence measure several hundred thousand counts per second which amounts to detecting and recording several thousand coincidences per second. The advantage over a collimator device lies mainly in the reduction of the radioactive dose to be used in the patient. The dose injected into the patient is 100 times less radioactive for an equivalent acquisition time.
An article by Mankoff, Muehllehner and Karp, published in Phys. Med. Biol., 1989, Vol. 34, No. 4, pages 437-456 and entitled "The high count rate performance of a two-dimensionally position-sensitive detector for positron emission tomography" mentions high-speed counting using coincidence. This article mentions the counting of two million counts per second using digital processing allowing a count to be processed in less than 250 ns. Nevertheless, at two million counts emitted per second, it is only possible to recover 62% of the counts which are however emitted experimentally to produce a coincidence for each count emitted. The other 38% correspond to counts not separated timewise by the gamma camera used. If we increase the count emission rate, the percentage of usable counts would be lower still.
If we transpose the device mentioned in the article above to medical use with a patient, the number of counts received and usable in coincidence must be divided by one hundred.
Whatever the case, increasing the number of counts emitted by the patient tends to make the counts received overlap and therefore reduces the number of valid counts beyond a certain number of counts emitted of around two million.
Examinations using gamma cameras are relatively long as they require the acquisition of images each requiring a high number of counts (to obtain a well defined image, several million counts may be required). A factor allowing a reduction in the examination time is to increase the number of counts to be taken into account per second. However, at present, the technology limits the number of counts to be accepted.
To sum up, the state of the art is as follows: